Now, three-way catalysts have been widely used as the catalysts adapted to purify nitrogen oxide (NOx), carbon monoxide (CO), and hydrocarbon (HC) in exhaust gases from motor vehicles, etc. The three-way catalyst is composed of a porous oxide carrier or support such as alumina, ceria, zirconia, ceria-zirconia solid solution, etc., which carries a noble metal such as platinum (Pt), rhodium (Rh), palladium (Pd), etc. The three-way catalyst oxidizes HC and CO and reduces NOx to purify the same. These reactions proceed most efficiently in such an atmosphere that oxidation components and reduction components exist in approximately equivalent, and consequently, in the motor vehicles on which the three-way catalysts are mounted, the air-to-fuel ratio has been controlled such that combustion is carried out at about the theoretical air-to-fuel ratio (stoichiometric ratio) (A/F=about 14.6±0.2).
The three-way catalysts, however, exhibit the problem that when an exhaust gas atmosphere shifts to a reduction side, sulfur oxide in the exhaust gases is reduced and emitted as H2S. Accordingly, in the method of publication of examined Japanese patent application No. 08-15554 (Document 1), oxide of Ni or Cu is added as a component of the three-way catalyst. Oxide of Ni or Cu forms SO3 or SO4 from SO2 in an oxidation atmosphere, and stores sulfur components as sulfides such as Ni2S3, ex. in a reduction atmosphere to restrain the emission of H2S.
Publication of unexamined Japanese patent application No. 2001-70754 (Document 2) discloses a device for purifying exhaust gases, wherein a three-way catalyst to which nickel oxide is added is disposed on a downstream side of a NOx strage reduction catalyst device which contains NOx strage components capable of decreasing the emission of NOx in the lean region. And Publication of unexamined international application No. 2004-523686 (Document 3) discloses a catalyst composite of which an upstream section contains a NOx strage component, and a downstream section contains a platinum component and a SOx adsorbing component such as nickel oxide, etc. However, in Documents 2 and 3, added nickel oxide deteriorates noble metal so that the original purification performance of the three-way catalyst by virtue of the platinum component, etc. lowers, and consequently, the purification of CO and HC becomes insufficient.